A standard metallized film capacitor widely known in the art is the wound capacitor. Wound capacitors are constructed by sandwiching a dielectric film such as polycarbonate, polypropylene or polyester film, between metal electrodes (e.g., vapor deposited metal film). Once formed, the combination dielectric/metal material is wound to form a capacitor. Some specific examples of wound capacitors are found in the following: U.S. Pat. No. 4,719,539 (Lavene), U.S. Pat. No. 4,685,026 (Lavene), and U.S. Pat. No. 5,614,111 (Lavene). Each of these U.S. patents are incorporated herein by reference.
The size of a capacitor is related to its breakdown voltage. The size of a metallized film capacitor is dictated by the thickness of its dielectric film. The thickness of the dielectric, in turn, is dictated by the required overall breakdown voltage of the capacitor. For instance, if a manufacturer cites a particular film as having a dielectric strength of 200 volts/micron and the capacitor design calls for a dielectric breakdown voltage of 400 volts, then the film may be 2 microns thick. Thus, the breakdown voltage of a capacitor depends on the dielectric strength and the thickness of the film.
When electrical current is passed through a wound film capacitor, thermal energy is generated raising the temperature of the capacitor. In large current applications (for example 7 amperes to 30 amperes), this thermal energy can be quite large and may lead to the deterioration of the capacitor. In some applications the thermal energy may even lead to an explosion.
Additionally, thermal energy may be increased if the capacitor is hermetically sealed, because the hermetic sealing may make it more difficult for the heat to be transferred to the exterior of the capacitor and be dissipated. It is known to place metal cover seals at the opposite ends of hermetically sealed capacitors, thereby increasing somewhat the transfer of thermal energy to the exterior of the capacitor. It is also known to provide perforations in these cover seals. The perforations permit outgassing to occur, when the capacitor is baked prior to sealing, thereby cleaning and drying the capacitor.
It is known to provide fault interrupters to prevent capacitors from overheating or exploding. U.S. Pat. No. 3,496,432 discloses a wound capacitor which forms gas when being overheated. The dielectric of the capacitor winding includes a foil of thermoplastic material with the property of contracting when heated. Thus, when the capacitor winding, upon heating, contracts in the axial direction, one of the metal layers is separated from the capacitor winding, so that electrical connection to the capacitor winding is interrupted.
U.S. Pat. No. 4,639,827 discloses a pressure sensitive fault interrupter for a film capacitor. The film capacitor has a dome-shaped diaphragm. When a fault occurs, pressure is developed within the capacitor as a result of the breakdown of the dielectric, thereby producing various gases. These gases fill the core of the capacitor and exert downward pressure on the diaphragm. The downward pressure changes the concave shape of the diaphragm into a convex shape, thereby breaking the electrical contact between the film capacitor and one of its tabs.
The present invention includes a fault interrupter (also referred to herein as a fuse or a thermal cutoff device) for a wound film capacitor which is different from any of the prior art, as described below.